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Torque then 90 degree

Can someone here give me a really simple explanation why they came up with torque to x then 90 degrees? Also how is that different to a higher torque setting. Sorry to sound dense but somewhere its just not making sense to me.

S

"According to the EPA each of us exhales 5.5 lbs of toxic pollution each day. So if you're green. Do your part.

Don't know, but I heard it differently : The ideal torque for any fastener is " 1) tighten until head snaps off, 2) back off 1/4 turn."

Actually I thought it was .... Torque until tight enough that you think it will break the tool(socket, hex or torx bit) of the next person who works on the car .. then add 50nm!! Yes ... I have a collection of broken snap-on tools waiting for my rep.

"According to the EPA each of us exhales 5.5 lbs of toxic pollution each day. So if you're green. Do your part.

Can someone here give me a really simple explanation why they came up with torque to x then 90 degrees?

Simply stated, it is because it is really not about torque - it is about having the right "stretch" of the fastener. Ideally, every time you saw a torque spec, you would see a "stretch" spec. You would install the fastener, torque it, and then remove it to check the stretch achieved. If not enough, then you would install and repeat. Ultimately, you would find the torque that achieved the right stretch. So, which would you publish?

In the torque to "x" and then 90 degrees scenario, the OEM has found that friction may distort torque readings at the desired limit. Therefore, a torque of "x" + 90 degrees gives the desired stretch on the fastener while eliminating friction and/or other variables.

Can someone here give me a really simple explanation why they came up with torque to x then 90 degrees? Also how is that different to a higher torque setting. Sorry to sound dense but somewhere its just not making sense to me.

S

The Accufab axel bolt kits give you a torqure range. Not the Ford method..

DBGT-
The Ford/Accufab question has been debated adnausum. Check the multiple previous threads. Bottom line both kits will WORK FINE. Like Mike says, if you are comfortable with a Ford dealer who has a tech who can change out the bolts to your satisfaction, do it! If not get the bolt kit free from Ford with your GT VIN and have a person of your choosing install the kit. Or buy the Accufab kit and have it installed by your preferred installer. Either way the problem is solved.

Actually nota4re has a pretty good explanation of torque-to-yield (TTY) bolts but let me amplify a bit on several of his points. Typical engineering practice is to design a bolted flange with as high a bolt preload as possible based on the bolt material. Typical recommendations are a bolt proof load equal to 90% of the material yield strength. Varying amounts of thread friction which can vary SIGNIFICANTLY by non-use of any lubricants (dry threads) or the use of engine oil, mouse milk, never-seize etc. can significantly alter the bolt load. Thus it it important to follow any factory assembly instructions carefully.

Thus if we design a bolt load to equal 90% of its yield strength (based on bolt tensile area) the bolt will never take a permenent "stretch". Measure the bolt length, install, torque to whatever torque value specified to take the bolt to 90% yield, untorque the the fastener and measure. The bolt will be identical in length as before the install. This is because the bolt was never loaded above its material yield point. This is true for the first torquing or multiple torquings thereafter.

I agree with nota4re, what we REALLY want to know is bolt STRETCH, but this is very difficult to measure instu. About the only application where you can easly measure this stretch is a rod bolt and there are instruments available to make this measurement. Since most applications cannot measure stretch directly we assume a thread friction (which varies with lubricant used) and calculate a bolt load.

When you torque a bolt above the material yield point (ie TTY bolts) you enter a portion of the material stress-strain curve that is not linear (meaning a given strain or bolt stretch no longer returns a linear amount of material stress or load) and the bolt load produced for a torque above yield is even less reliable at predicting actual load developed by the fastener. For this reason, torquing specs in this plastic/non-linear region of the bolt material strength are usually specified as an initial amount of torque to take the load up to near yield and then an additional angle-of-turn afterward to load the bolt into the non-linear bolt strength region.

It was stated, and is true, the six Accufab outer diameter fasteners on our rear axle half shafts come with standard torque callouts (resulting in below material yield stress) whereas the Ford specified bolts are TTY. My preference for this bolted assembly is the TTY bolt as for a given bolt size a higher preload can be achieved with a TTY fastener. Again though, both kits will adequately address the previous inner bolt failures.

So now that I am significantly less confused, more or less than before, but not as much as I was yesterday, but significantly more than I was the day I was born, and yet less than the moment that I got this cerebral hemorrage trying to figure out which bolt to use on my car, but not as much as I was before I asked the remaining question that I dare to ask. Can a TTY bolt be reused if it is over torqued due to operator error, or rather to say the technician that comes into review some previous work removes the bolt without knowledge of the previous installation?

I think I am going to go buy a set of accufabs and mix them with the ford kit just to play russian roulette.

"According to the EPA each of us exhales 5.5 lbs of toxic pollution each day. So if you're green. Do your part.